This invention relates to amplifiers, and more particularly to switchable power amplifiers.
As is known in the art, power amplifiers are used in a wide variety of applications. One such application is in telephones, such as battery operated cell phones, to amplify radio frequency (RF) signals.
As is also known in the art, such RF amplifiers are typically biased to a proper direct current (dc) operating point using a current mirror 9. One such an arrangement is shown in FIG 1. Here, the amplifier includes an RF grounded emitter output transistor QRF. The base of the RF transistor is fed input RF signals. The base of the output transistor is also coupled to a switching transistor, Q1. A diode-connected transistor Q2 (i.e., p-n junction) is connected between ground and the base of the RF output transistor QRF. When such diode-connected transistor Q2 conducts, a fixed, dc voltage is produced across it thereby fixing the dc voltage at the base of the RF output transistor QRF. Thus, when conducting the diode-connected transistor Q2 provides a fixed reference voltage at the base electrode of the RF output transistor QRF. The RF transistor QRF is biased to a fixed dc operating point by providing a fixed dc current IBIAS through the collector-emitter thereof. Here, the dc current is produced by applying a fixed current through the conducting diode connected transistor Q2. The fixed dc current is supplied by a current source, i.e., a high impedance device, here represented by a resistor. The current source here produces the current indicated I. Thus, the amount of dc bias current required for the RF output transistor establishes the amount of current I provided by the current source. That is, the diode-connected transistor and the RF output transistor QRF are arranged as current mirror 9 with the current through the collector of the RF transistor mirroring the current fed to the diode-connected transistor Q2 from the current source.
When it is desired to switch the RF output transistor QRF off, the transistor Q1 is driven on by a control voltage fed to the base thereof. Thus, the on condition of transistor Q1 places a low voltage at the base of RF output transistor QRF. The current I from the current source now passes through the conducting transistor Q1. Transistor Q1 must sink the current source current I which may be as high as 5-10 milli-amps in some applications resulting in excessive battery power drain.
Another circuit is shown in FIG. 2. Here, the current mirror 9xe2x80x2 is provided by a pair of transistors Q2A and Q2B and the RF output transistor QRF. Here, the current mirror 9xe2x80x2 is sometimes referred to as an enhanced current mirror. With an enhanced current mirror the emitter follower transistor can supply extra base current as required when the RF transistor is driven under high power, large signal conditions.
Here again a fixed dc voltage is produced at the base of the output transistor QRF. The dc voltage is coupled to the base of the RF transistor as a fixed dc voltage and thereby provides a proper dc bias current for the RF output transistor. The amount of dc bias current required for the RF output transistor is established by the amount of current I from the current source, here again represented by a resistor. Here again, when it is desired to switch the RF output transistor off, the transistor Q1 is driven on by a control voltage fed to the base thereof. Thus, the on condition places a low voltage at the base of RF output transistor QRF. The current I from the current source now passes through the conducting transistor Q1. Transistor Q1 must sink the current source current I which may be as high as 5-10 milli-amps in some applications resulting in excessive battery power drain.
In accordance with the present invention, an amplifier circuit is provided. The amplifier circuit includes a current mirror circuit. The current mirror circuit includes: a first current source for producing a reference current; an output transistor having an input electrode and an output electrode; and a current gain device connected between an output of the first current source and the input electrode of the output transistor. A bias current is produced through the output electrode of the output transistor, such bias current being a function of the reference current produced by the first current source. A second current source has an output coupled to an input of the current gain device. The second current source provides a current which is a fraction of the reference current. A switching transistor has an output electrode coupled to: (1) an input of the current gain device; and, (2) an output of the second current source. The switching transistor: (1) sinks the current from the second current source in response to an input signal fed to such switching transistor inhibiting current from the second current source to pass to the current gain device and thereby remove the bias current for the output transistor driving the output transistor to a non-conducting condition; or (2) enables the current from the second current source to pass to the current gain device in response to the input signal fed to such switching transistor driving the output transistor to a conducting condition.
In one embodiment, the gain device comprises a transistor.
In one embodiment, the gain device transistor is driven into saturation when the switching transistor enables the current from the second current source to pass to the current gain device in response to the input signal fed to such switching transistor driving the output transistor to a conducting condition.
In one embodiment, the gain device transistor comprises a bipolar transistor.
In accordance with the invention, a transistor is provided having the collector-emitter coupled between the fixed voltage at the output of the current source and the base of the RF transistor. This transistor has its base coupled to a second current source and to a switch for such transistor. When the RF is to turn on, the current from the second current source is fed to the base of the transistor to bias the transistor into saturation with the new circuit producing a fixed dc voltage at the base of the RF transistor and therefore provides a fixed dc bias current for the RF transistor. The amount of dc bias current required for the RF transistor establishes the amount of current required for the current source. Because the transistor is in saturation, the second current source is only a small fraction (I/beta) the amount of current required by the first current source to provide the requisite dc bias current for the RF transistor. When the switch turns the RF transistor off, the smaller current from the second current source is sunk by the switch.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.